At present, the range of application of liquid crystal compounds is expanded wider and wider, and liquid crystal compounds can be used in various types of displays, electro-optical devices, sensors, etc. There are a great variety of liquid crystal compounds used in the above-mentioned display field, wherein nematic liquid crystals are used most extensively. Nematic liquid crystals have been used in passive TN and STN matrix displays and systems having a TFT active matrix.
As for the application field of the thin film transistor techniques (TFT-LCD), although the market in recent years has been very huge and techniques also become mature gradually, requirements on display techniques continuously increase as well, especially in aspects of achieving a quick response, reducing the driving voltage to reduce the power consumption, etc. Liquid crystal materials, as one of important optoelectronic materials for liquid crystal displays, play an important role in improving the performance of a liquid crystal display.
As liquid crystal materials, they need to have good chemical and thermal stability and stability with respect to electric fields and electromagnetic radiations. Moreover, as liquid crystal materials used for thin film transistor techniques (TFT-LCD), they not only need to have the stabilities as mentioned above, but also should have properties, such as a broader nematic phase temperature range, a suitable birefringence anisotropy, a very high electrical resistivity, a good anti-ultraviolet property, a high charge retention rate, a low vapor pressure, etc.
As for the application of dynamic picture displays, elimination of ghosting and trailing of display pictures the liquid crystal is required to have a very fast response speed, and therefore the liquid crystal is required to have a lower rotary viscosity γ1; in addition, in order to reduce the equipment energy consumption, the driving voltage of a liquid crystal is desired to be as low as possible.
In order to achieve a high quality display, a high contrast ratio is another important performance parameter of a liquid crystal display, and the contrast ratio is in fact a specific value of luminance, defined as: the luminance of a white picture (when at the brightest) divided by the luminance of a black picture (when at the darkest) in a dark room. More accurately, the contrast ratio is namely a numerical value obtained by, from the saturations when the white signal is at 100%, subtracting that at 0% and then dividing same by the white value (the white signal at 0% is actually black) at 0% with Lux (illuminance, i.e., lx, the lumen value per square meter) as the unit of measurement. Contrast ratio is a value by dividing the blackest luminance unit by the whitest luminance unit. Therefore, the brighter the white color and the darker the black color, the higher the contrast ratio. Contrast ratio is an important parameter of a liquid crystal display, and under a reasonable luminance value, the higher the contrast ratio, the more abundant the color gradation displayed thereby.
With regard to these displays, a new liquid crystal composition with improved performance is required. A high contrast ratio is a property that such a mixture must have. In addition, such a mixture should further have an appropriate dielectric anisotropy (Δε), an optical anisotropy (Δn) and a threshold voltage.
Therefore, there is a need for a new liquid crystal composition having suitable properties with regard to practical applications, such as a broader nematic phase range, an appropriate dielectric anisotropy (Δε), optical anisotropy (Δn) and operating voltage, a low rotary viscosity, and in addition, having a high contrast ratio property, especially where the contrast ratio has a low change rate at a low temperature.